Clear the National Guard training yard of ghouls
Fallout 4 Guide

It is recommended that you complete this quest before the Battle of Bunker Hill. Talk to Deb, and she will task you with clearing the National Guard Training Yard of ghouls.

Clear the exterior and interior and of the facility of all ghouls - you do not need to enter the garrison, nor the storage

You need to kill ghouls in the building and around it, they have been marked on the radar. You receive information on completion of the quest, after you kill them all. Return to Deb for the reward. You do not need to worry about turrets, only ghouls are your target.